This invention relates to roll coaters and more particularly to a roll coater in which the positioning of the metering roll may be controlled with high precision.
The invention relates generally to the type of coater as shown in the patent of Fridolph, U.S. Pat. No. 2,681,636 of June 22, 1954, in which a metering roll is movable on guideways into and out of engagement with a coating transfer roll which, in turn, applies a metered quantity of coating to a web of paper threaded between the coating transfer roll and a backing roll. Cylinder motors have been used in roll coaters to provide for rapid movement of the rolls and to bias the metering roll against adjustable stops spacing it from the transfer roll, all as shown in Fridolph above.
It is particularly desirable to be able to move the metering roll a substantial distance out of contact with the coating transfer roll from a closely controlled metering position to a retracted position, for safety's sake, and for cleaning of the machine. It is of prime importance in precision coaters to provide an arrangement for returning the metering roll to its predetermined relationship with the coating transfer roll.
For example, metering rolls and the supporting bearing assemblies, can be made with a total indicated runout of near zero. Using such precision roll coating equipment, the gap between that roll and the coating transfer roll can be as small as 0.000750" (750 micro-inches), and the tolerance required of the metered coating film on the web may be in the order of plus or minus 5% thickness or 0.000075" (75 micro-inches). Thus, a mechanism which has the capability of operating at such close tolerances, must be one in which the same can be established and reestablished from time to time after the rolls have been moved to a retracted position. Any substantial shock loading created when the metering roll bearing housings contact the spacer blocks in the guideways separating these bearing housings from those of the coating transfer roll, can result in disorientation of the rolls and the supporting structure to a degree geater than the plus or minus 5% thickness tolerance in the 75 micro-inch range. There is accordingly a need for an actuating mechanism which may be operated for quickly opening the metering roll from the transfer roll, moving the metering roll to a substantial retracted position, to provide for a 4" gap or the like, and for returning the same to a precisely controlled coating position without undue shock, so as to maintain a predetermined optimum coating condition.
Further, depending on the coating material being metered, the speed of operation, and the width of the coater, there are instances when the metering force actually causes the metering roll to deflect by an amount greater than the coating thickness. In such cases, a cross-axis control or crown must be put on the metering roll to compensate for such deflection. However, in conventional coater apparatus, the closing movement is not controlled in such a manner as to allow a metering force to be built up or generated as the metering roll approaches its metering position, and thus problems with the crown actually contacting the transfer roll surface have occurred. For this additional reason there exists a requirement for providing greater control in the closing movement of the metering roll with respect to the transfer roll.